Excerpt from a /r/bogleheads comment on the subject:

If you had invested money the day Alan Greenspan uttered the now famous phrase “Irrational Exuberance”, held it until the dotcom bust, and sold at the bottom, you still would have come out ahead.

Knowing there’s a bubble does nothing to help you understand what will happen to the bubble. In order to do that, you need to know:

1. That there is a bubble.

2. How high the bubble will go.

3. When the bubble will pop.

4. When the bubble bottoms out.

Nobody knows 2, 3, and 4.